Wet strength paper containing polyamide



United States Patent Ofitice 3,019,156 Patented Jan. 30, 1962 3,019,156WET STRENGTH PAPER CONTAINING POLYAMIDE Lennart A. Luudberg, Stamford,Conn., and Lucius H. Wilson, Valdosta, Ga., assignors to AmericanCyanamid Company, New York, N.Y., a corporation of Maine No Drawing.Filed Apr. 29, 1957, ser No. 655,488 8 Claims. (Cl. 162-468) The presentinvention relates to Wet-strength paper. More particularly, theinvention relates to wet-strength paper cbntaining a normallywater-soluble cationic linear methylenebisacrylamide-alkylene-polyaminecondensate as agent imparting wet strength. The invention includes thepapermaking processes involved.

In the past the manufacture of Wet-strength paper has been successfullyperformed on a large scale by the addition of Water-soluble cationicresinous condensates to aqueous slurries of papermaking fibers. Thecondensates are substantively adsorbed by the fibers while in aqueoussuspension and bond or spot Weld the fibers together when sheeted anddried.

In the past, effective adsorption of the bonding agent has generallyrequired that the cellulose pulp be acid during adsorption of the resin,and development of satisfactory wet strength has generally required thatthe agent have a substantial content of formaldehyde which acts ascross-linking agent when the fibers are sheeted and dried.

The present invention provides a new papermaking process which may becarried out at an alkaline pH, and provides a new form of wet-strengthpaper which need not contain any formaldehyde at all.

The discovery has now been made that the normally water-soluble cationicmethylenebisacrylamide-polyamine condensates are particularlyadvantageous 'as Wet-strength agents in the manufacture of paper. In thefirst place, the condensates are adsorbed by cellulose fibers in aqueoussuspensions not only at normally acid pH values but at alkaline pHvalues as Well. In numerous instances we have been successful in makingWet-strength paper at pH values as high as 9.0, thus demonstrating thatthe acid corrosion problem can be eliminated. In the second place, theagents need contain no formaldehyde or inorganic anionic material sothat they are particularly suitable for the manufacture of photographicand electrical paper. Thirdly, the condensates are non-thermosettingunless otherwise desired. As a result, the paper need not be heated atany particular temperature for development of substantial Wet strength.Finally, the condensates are readily converted to three-dimensionalthermosetting form, for example, by reaction with formaldehyde, as aresult of which paper of still better wet strength can be prepared onthe alkaline side.

The paper of the present invention is thus composed of a Waterlaid Webof cellulose fibers bonded together by a small but effective adsorbedamount of a normally watersoluble cationic linearmethylenebisacrylamide-polyfunctional amine condensate. If desired, thecondensate in its normal form may be thermosetting resulting from acombined content of formaldehyde. The paper may have an alkaline pH.

Broadly, paper is manufactured according to the present invention byforming a dilute aqueous suspension of papermaking cellulose fibers,adding thereto a small but effective amount of a water-soluble cationicmethylene bisacrylamide-polyfunctional amine condensate as wetstrengthagent thereby adsorbing said condensate on said fibers, sheeting thefibers to form a waterlaid web, and drying the web. The pulp may have anacid or alkaline pH during the operation. Where the condensate has beenpre-reacted with formaldehyde to render it thermosetting, drying atF.-260 F. for /2 to 5-10 minutes is generally about sufiicient todevelop the maximum strengthening properties of the resin.

More in detail the paper of the present invention is manufactured byforming an aqueous slurry of beaten (i.e., hydrated) cellulose fibers ata normal consistency between about 0.5 and 5%. There is then added anappropriate amount of the wet-strength condensate with circulation ofthe pulp for a few minutes to allow uniform adsorption to occur. Thepulp is then sheeted and dried in normal fashion.

In the process the pH of the pulp during addition of the condensate maybe any desired value within the normal limits of about pH 4 and pH 10and there is no necessity to change the pH of the pulp after addition ofthe resin, although, if desired this may be done. Better wet strength isgenerally obtained when the pH of the pulp during the adsorption step ison the alkaline side. If desired, the fibers may be sized by addition ofrosin size and alum preferably added prior to the Wet-strength agent, inwhich case to prevent desorption of the size the pH of the pulp shouldnot be made alkaline thereafter.

It is further within the scope of the invention to add fillers,opacifiers or colorants to the slurry prior to sheet formation. Thepresent invention permits materials of these types such as Ultramarine,calcium carbonate and magnesium carbonate (Raffold) to be added.

The amount of condensate added in any one instance is determined chieflyby the amount of wet strength which it is desired the paper shouldpossess. A noticeable improvement results from the addition of at least0.1% of the resin based on the dry weight of the fibers. The wetstrength imparted tends to level off When more than about 5% by weightof the agent is added. In practice, we prefer to add between about /2and 3 of the agent based on the dry weight of the fiber as in this rangesubstantial wet strength is imparted with about maximum efiiciency inthe utilization of the agent. The condensate is rapidly adsorbed,generally in less than 15 minutes.

The condensates may be prepared by reacting methylenebisacrylamide orequivalent material with a watersoluble alkylenediamine orpolyalkylenepolyamine so as to form a linear condensate. In general,sufiicient of the alkylenediamine or polyalkylenepolyamine should beused so that the methylenebisacrylamide reacts preferentially with theterminal primary amino groups thereof thereby favoring formation oflinear chains. There may thus be used between about 0.5 mol to 2.0 molof the methylenebisacrylamide per mol of 'alkylenepolyamine orpolyalkylenepolyamine and this ratio is constant whether or not thepolyamine is a simple alkylenediamine such as ethylenediamine,trimethylenediamine, etc., or triethylenetetramine,tetraethylenepentamine, 3,3'-iminobispropylamine or the condensationproducts of the foregoing amines with small amounts ofalkylenedichlorides or epichlorohydrin so as to build up largelyessentially strait chain polyamines. The alkylene groups referred tocontain not in excess of six carbon atoms.

As methylene-bisacrylamide, there may be used methylenebisacrylamideitself and water-soluble substantially non-ionic derivatives thereofincluding methylenebismethacrylamide andN,N-dimethylmethylenebisacrylarnide. condensate imparting best wetstrength are gener- 'ally made by reacting the polyamine andmethylenebisacrylamide in substantially equirnolecular ratio, followedby addition of a chain stopper as the point of gelation is neared. Thisprocedure constitutes a simple means for developing a condensate whichis substantially linear and water-soluble though of large molecularsize.

The polymer syrups obtained are normally alkaline and the polymertherein may be converted to thermosetting form by reaction withformaldehyde. Generally between about to 1 mol of formaldehyde is addedper aldehyde-reactive nitrogen atom present, which is sufiicient tocause extensive cross-linking when the polymer is thermocured on thefibers.

The reaction between the methylenebisacrylamide and the polyamineproceeds rapidly at moderate temperatures and may be terminated byadding a secondary alkyl amine when molecular growth has proceeded tothe point short of gelation where the condensate is sufiicientlycationic, the secondary amine acting as chain stopper. The point atwhich the condensate exhibits cationic properties may be determined bythe well-known electrophoresis method and in practice occurs when thereaction mass has become quite viscous. The chain stopper is added atsuch point as to permit production of a highly viscous reaction productwhich, however, is water-soluble. Alternatively, the reaction may behalted by the addition of a large amount of water with rapid cooling.When less than about 0.85 mol of methylenebisacrylamide is used per molof polyalkylenepolyamine or alkylenepolyamine, cross-linking does notoccur and a chain stopper need not be used.

The invention does not primarily depend upon the particular rawmaterials employed or the ratio in which they are reacted. A number ofcondensation products of the type described which are suitable for thepractice of the present invention are disclosed in application SerialNo. 623,555, filed on November 21, 1956, by L. A. Lundberg, now PatentNo. 2,934,528.

The invention will be more particularly described by the examples whichfollow. These examples illustrate embodiments of the invention and arenot to be construed as limitations thereon.

Example 1 The following illustrates the preparation of a watersolublecationic 1:1 molar ratio methylenebisacrylarnidealkylene-polyaminecondensate employing ethylenediamine as the polyamine and diethylamineas the chain stopper.

In a round-bottomed flask equipped with thermometer, reflux condenserand stirrer were placed 30.8 gm. of methylenebisacrylamide (0.2 mol),12.4 gm. of ethylenediamine (0.2 mol) and 63.8 gm. of water. The mixturewas heated slowly to 70 C. and held for several minutes at thistemperature until the syrup became very viscous. There was then added3.7 gm. of diethylamine (0.05 mol) and heating continued for half anhour on a steam bath to a maximum temperature of 84 C. The product was aclear essentially linear water-white syrup which remained stable onstanding.

The condensate was tested by adding 3.0% to a 0.6% aqueous dispersion ofbleached kraft pulp (resin solids based on the dry weight of thefibers), adjusting the pH of the suspension to 9.0 with sodiumhydroxide, allowing the suspension to stand with gentle agitation forminutes during which adsorption of the condensate occurred on thefibers. The pulp was sheeted on a Nash handsheetmachine. The sheets werepressed between blotters and dried for 2 minutes at 240 F. on a Nobleand Wood handsheet dryer. Additional sheets were made in the same wayand given an extra cure by hanging for 10 minutes in an oven at 260 C.

The first set of sheets had a wet strength of 4.6 lb./ in. and thesecond set (with extra cure) had a wet strength of 5.2 lb./ in. Thebasis weight of the sheets was 47.9 lb. per 25" x 40/500 ream.

Example 2 The procedure of Example 1 was repeated except that 29.6 gm.(0.2 mol) of triethylenetetramine was used in place of theethylenediamine, the amount of water was decreased to 61.4 gm. and theamount of diethylamine increased to 7.3 gm. (0.1 mol).

The first three reagents were mixed and heated at 50 C. for sevenminutes, after which the diethylamine was added. A similar condensateresutled, which was tested in accordance with Example 1. The paper wastested in similar manner yielding wet-strength values of 4.6 lb. and 4.8lb./in. at a basis weight of 49.9 lb.

Example 3 The procedure of Example 2 was repeated except that the amountof Water initially added was increased to 184 gm. and the mixture heatedfor 10 minutes at 50 C. at which point the syrup became very viscous. Nochain stopper was employed and instead 365 gm. of water at 50 C. wasadded and the mixture allowed to cool.

The syrup was tested in the same manner as the foregoing syrup and gavewet strength values of 4.7 lb. and 5.0 lb. at a basis weight of 49.4 lb.under conditions of regular and extra cure respectively.

Example 4 In the foregoing examples the condensate was.nonthermosetting.

The following illustrates the preparation of a thermoseting condensateand the effect of slow drying on the strength of the wet-strength bonddeveloped. The procedure for the preparation of the condensate ofExample 2 was repeated, after which 32.7 gm. (0.4 mol) of 37% aqueousformalin solution was added 10 minutes after the diethylamine had beenrun in. The formaldehyde was reacted with the condensate over 15 minutesat elevated temperature rendering the condensate thermosetting and thesolution cooled. The condensate was tested according to the method ofExample 1. The paper had a wet strength of 5.3 lb. at a basis weight of48.6 lb.

Example 5 The procedure of Example 2 was repeated using 26.2 gm. (0.2mol) of 3,3-iminobispropylamine in place of the triethylenetetramine.The wet strength of the paper was 5.5 lb./in. at 49.3 lb. basis weight.

We claim:

1. Wet-strength paper composed of waterlaid interfelted cellulose fibersbonded together by a small but effective adsorbed content of a normallywater-soluble cationic linear methylenebisacrylamide-alkylenepolyaminecondensate as agent imparting said wet strength.

2. Paper according to claim 1 wherein the condensate is a normallythermosetting methylenebisacrylamide-alkylenepolyamine-formaldehydecondensate in thermoset form.

3. Paper according to claim 1 having an alkaline pH.

4. Paper according to claim 1 wherein the alkylenepolyamine isethylenediamine.

5. Paper according to claim 1 wherein the alkylenepolyamine is3,3-iminobispropylamine.

6. A process of manufacturing paper of improved wet strength whichcomprises forming a dilute aqueous suspension of papermaking cellulosefibers, adding thereto an effective amount of an aqueous solution of acationic watersoluble methylenebisacrylamide-alkylenepolyaminecondensate as wet-strength agent, whereby said condensate is adsorbed bysaid fibers, sheeting the fibers to form a Waterlaid web, and dryingsaid web.

7. A process according to claim 6 wherein the suspension is alkalineduring and subsequent to adsorption of the condensate.

8. A process according to claim 6 wherein the condensate is athermosetting cationicmethylenebisacrylamidealkylenepolyamine-formaldehyde condensate and theWeb is heated at a temperature between about 180 and 260 F. for /2 to 5minutes to dry the web and thermoset the condensate thereon.

References Cited in the file of this patent UNITED STATES PATENTSKleiner et al June 30, 1953 Jen Nov. 6, 1956 Lundberg et a1 June 4, 1957Lundberg et a1 July 30, 1957 Aycock et a1 Apr. 22, 1958

6. A PROCESS OF MANUFACTURING PAPER OF IMPROVED WET STRENGTH WHICHCOMPRISES FORMING A DILUTE AQUEOUS SUSPENSION OF PAPERMAKING CELLOUSEFIBERS, ADDING THERETO AN EFFECTIVE AMOUNT OF AN AQUEOUS SOLUTION OF ACATIONIC WATER-SOLUBLE METHYLENSBISACRYLAMIDE-ALKYLENEPOLYAMINECONDENSATE AS WET-STRENGTH AGENT, WHEREBY SAID CONDENSATE IS ADSORBED BYSAID FIBERS, SHEETING THE FIBERS TO FORM A WATERLAID WEB, AND DRYINGSAID WEB.